1. Field of the Invention
The present invention relates to an optical switching system and an optical switching method used in the field of an optical communication system, and particularly relates to an optical switching system and an optical switching method wherein a wavelength division type technique and a space division type technique are effectively combined.
2. Related Art
In the recent years, optical switches for switching an optical signal as it is without converting it to an electric signal have been gaining attention in the field of an optical communication system, as a technique for realizing a large capacity switch which is hard to realize with an electric switch. Optical switches include a space division type, a wavelength division type, a time division type, etc. Further, there are techniques in which those types are combined, such as a wavelength/space division type, a wavelength/time division type, etc. Some of those combined types will now be explained.
FIG. 10 is a block diagram of an ordinarily conceivable wavelength division type optical switching system. This wavelength division type optical switching system comprises optical wave dividers 51, optical wave multiplexers 52, and space division type optical switches 53. The optical wave dividers 51 are provided to input light transmission paths Input 1 to Input k respectively, and divide an optical signal in accordance with frequencies. One space division type optical switch 53 includes (kxc3x97k) number of space division type optical switches prepared for n number of lights xcex1 to xcexn obtained by division and each having its own wavelength. The optical wave multiplexers 52 are provided to output light transmission paths Output 1 to Output k respectively, and multiplex optical signals each having its own wavelength. With this structure, the wavelength division type optical switch needs to comprise an optical wave divider 51 and optical wave multiplexer 52 for every input or output optical fiber, and further to comprise a space division type optical switch for every wavelength. Therefore, a lot of optical fibers are necessary, and this is not suitable for miniaturizing the device.
On the other hand, Unexamined Japanese Patent Application KOKAI Publication NO. H11-243564 discloses an optical switch having intermediate function and structure between a space division type optical switching system for switching light paths on an optical fiber by optical fiber basis, and a wavelength division type optical switching system for switching light paths on a wavelength by wavelength basis. This optical switch will be explained with reference to FIG. 11.
As illustrated in FIG. 11, this optical switch comprises a plurality of branching units 54, insertion units 55, a first space division type optical switch 56, a second space division type optical switch 57, and a third space division type optical switch 58. The branching units 54 are provided to input light transmission paths Input 1 to Input k1, respectively. The branching units 54 select a signal having a predetermined wavelength from an input wavelength-multiplexed optical signal, output the selected signal to the second space division type optical switch 57, and output the remaining optical signal to the first space division type optical switch 56. The second space division type optical switch 57 routes the branched optical signal to a predetermined output light transmission path in order to output the branched optical signal. On the other hand, the insertion units 55 multiplex an optical signal input by the third space division type optical switch 58 with the optical signal input by the first space division type optical switch 56, and after this, transmit the resultant optical signal to output light transmission paths Output 1 to Output k1.
The optical switching system disclosed in Unexamined Japanese Patent Application KOKAI Publication No. H11-243564 is simply structured, and has an advantage that light transmission paths can be shared. However, since a wavelength-multiplexed signal needs to pass through the branching units 54 and/or insertion units 55, there is a problem that the loss of the optical signal is large. Further, in order to compensate for the loss of an optical signal, it is necessary to provide an optical amplifier, or to perform optical/electric/optical (hereinafter, referred to as O/E/O) conversion. However, there has been a problem that using such methods makes a device large-sized.
An example of an optical switching system employing a wavelength/space division type optical switch is disclosed in Unexamined Japanese Patent Application KOKAI Publication No. H11-275614. FIG. 12 is a schematic block diagram of this device. This optical switching system comprises optical multiplexers 61, an optical wavelength router 62, a space division optical switch 63, and optical wavelength selectors 64. The optical multiplexers 61 multiplex optical signals having different frequencies. The optical wavelength router 62 divides the optical signal multiplexed by the optical multiplexers 61 in accordance with wavelength, and outputs the divided signals to the space division optical switch 63. The space division optical switch 63 routes each optical signal in accordance with wavelength. Further, the optical wavelength selectors 64 each select an optical signal having a desired wavelength and output the selected optical signal. This optical switching system can increase or decrease the number of wavelengths to be multiplexed. This enables the number of wavelengths to be multiplexed to be optimized. Accordingly, effects that (1) the amount of hardware such as optical gate switches, etc. necessary for an optical switching system can be minimized, and that (2) in a case where the number of wavelengths to be multiplexed is reduced, optical amplifiers used as optical gate switches, etc. do not reach saturation, can be obtained.
However, the optical switching system disclosed in Unexamined Japanese Patent Application KOKAI Publication No. H11-275614 requires all data which will pass through this optical switching system to pass through the gate switches so as to be switched inside tile device. Accordingly, there is a problem that the device becomes large-sized.
On the other hand, as a large-sized optical switching system, one employing an MEMS (Micro-electromechanical System), etc. is developed (NIKKEI ELECTRONICS Jan. 29, 2001 issue (NO. 788) pp. 146-167). With this optical switching system employing the MEMS, a small-sized optical switching system can be structured, even though there are a relatively large number of switch ports. However, since insertion loss of light caused in the large-sized optical switch is large, it is required to use an optical amplifier conversion device, etc. Therefore, there is a problem that the optical switching system as a whole becomes large-sized.
The above optical switching system can serve well under certain usage conditions, or with some restrictions. However, various problems might be caused, when considering increase in the traffic and demand for miniaturization of the optical switching device which might be expected in the future. That is, even in a case where conducting optical switching operation where large amount of data are processed, it will be inevitable in the future to miniaturize the optical switching system, increase the number of switch ports, and reduce the insertion loss of an optical signal. Especially, miniaturizing the device and increasing the number of switch ports are contradictory to each other. With regard to this, an efficient method which minimizes the need for performing O/E/O conversion now performed in the existing devices is demanded.
Further, as the amount of information handled in a network increases, the size of processing sections of an electronic circuit is enlarged, and this increases the cost of node devices.
It is an object of the present invention to solve the above-described problems of the prior arts.
It is another object of the present invention to provide an optical switching system which is adaptable to the future possible increase in the amount of communications through optical communication.
It is still another object of the present invention to provide an optical switching system which can satisfy miniaturization of the device, increasing the number of switch ports, and reducing insertion loss of an optical signal all at a same time.
To accomplish the above objects, according to the present invention, there is provided an optical switching system comprising:
a first space division type optical switch which is positioned at an input side, and performs switching on an optical fiber by optical fiber basis;
a second space division type optical switch which is positioned at an output side, and performs switching on an optical fiber by optical fiber basis; and
a wavelength division type optical switch which receives a wavelength-multiplexed signal through all optical fiber, performs wavelength division, switching on a wavelength by wavelength basis, and wavelength multiplexing, and outputs a wavelength-multiplexed signal through an optical fiber,
wherein some of optical fibers which are output from the first space division type optical switch are directly led to the second space division type optical switch, and the others are led to the wavelength division type optical switch, and optical fibers which are output from the wavelength division type optical switch are led to the second space division type optical switch.
Preferably, the wavelength division type optical switch may include an add port and/or a drop port for inputting or outputting a signal having a single wavelength.
More preferably, the first and second space division type optical switches may be constituted by a movable optical fiber type mechanical optical switch or a mirror type switch, and a switching unit of the wavelength division type optical switch may be constituted by a mirror type switch or an electric optical switch.
It is extremely difficult to satisfy miniaturization of a device of an optical switching system, increasing the number of ports, and reducing the loss of an optical signal at a same time. With regard to miniaturization of the optical switching system, simply to reduce the size of elements such as optical switches is effective. However, even more reliable way is to take advantage of characteristic of wavelength multiplex transmission. More than half of the optical signals to be processed by an optical switching system employing wavelength multiplex transmission are ones that merely pass through the optical switching system. Despite this, those optical signals that just pass through the switching system are unnecessarily processed. For example, all optical signals having different wavelengths are subjected to O/E/O conversion. This unnecessary processing has brought about the large-sizing of the device and increase in loss of an optical signal. With this situation taken into consideration, a research was made about an optical switching system employing wavelength multiplex transmission. As a result, a fact was found out that it is important to do away with wavelength division and wavelength multiplexing of signals that simply pass through the optical switching system while being wavelength-multiplexed on an optical fiber by optical fiber basis, in order to miniaturize the device, reduce the loss of an optical signal, and further increase the number of ports.
According to the optical switching system of the present invention, signals that simply pass through the switching system while being wavelength-multiplexed on an optical fiber by optical fiber basis only pass through the space division type optical switches, and optical signals that should be processed on a wavelength by wavelength basis pass through the wavelength division type optical switch which employs wavelength division and wavelength multiplex.
Further, another fact was found out that it is important to provide an add port and drop port to the wavelength division type optical switch, in order to apply the optical switching system according to the present invention in various ways, regardless of a basis system, a metro system, and a subscriber system.